Mobile telephones, in common with many radio systems up-convert a signal to be transmitted from a base band to the transmission frequency. A relatively wide spread of transmission frequencies are supported by the mobile telephone and consequently the transmission oscillator and local oscillator provided within such a telephone need to be tuneable over a relatively wide range of frequencies.
In general, if it is desired to tune a voltage controlled oscillator, VCO, over a relatively wide frequency range, then a relatively large constant of proportionality KVCO between the oscillator output frequency and the oscillator input control voltage is required. The use of a large KVCO enables the tuning range to be traversed quickly. This means that the local oscillator can be rapidly moved between frequencies and locked to the new frequency. There is, however, a penalty to be paid for this ease of tuning. Any noise appearing on the control voltage has potential to appear in the oscillator's output spectrum. This noise can result in fluctuations of the phase of the oscillator's output from the phase of an ideal sinusoid having the same frequency as the nominal frequency of the oscillator. These deviations amount to phase noise at the output of the oscillator.
The GSM standard for mobile telephones places limits on the phase noise that can be permitted in the transmission envelope of a mobile telephone. The limits are mandatory in that devices falling outside of these limits will not be accredited for use. The production of phase noise in the local oscillator or the transmission oscillator of the mobile telephone could easily cause the output signal's power density away from the nominal transmit frequency to exceed the permitted transmission power envelope. As a result, the voltage controlled oscillators of a mobile phone have typically been expensive discreet devices.
One way to address the phase noise problem is to have the voltage controlled oscillator with a much lower KVCO. Thus any noise on the oscillator control voltage has a proportionally reduced effect on the oscillator output frequency. However this, whilst reducing phase noise, makes it more difficult to tune the oscillator over a wide operating range.
Within a mobile telephone the VCO is included within a phase locked loop (PLL). Phase locked loops are well known to a person skilled in the art. A reference signal and a signal derived from the output of the VCO are compared by a phase detector. The output of the phase detector is then transformed (typically by low pass filtering) into a control signal for the VCO.
For some oscillator topologies variations in oscillator amplitude give rise to variations in oscillator frequency and also affect KVCO and hence the loop gain of the PLL. This can make the design and control of a fast response frequency agile low phase noise VCO and associated circuitry difficult.
However, for other reasons the designer may actually wish the oscillator amplitude to be set to different target values for different modes of operation of the telephone. Thus oscillator amplitude may be larger during a transmit period compared to a receive period as phase noise requirement in the transmit process are more stringent.